Using it is simple: just size the window to the area that you want to focus on. Once you've got it right, hit the ESC key, and the rest of the screen will go to black. Hit ESC again to return your view to normal.
There are some other general options, like activating and deactivating certain screen elements, for example graphic transition effects.
Zorro is an application to check out both for its simplicity as well as its helpful purpose.